For instance, the bellows (bellows formed from a base body whose side wall has a bellows shape) used for the vacuum capacitor is provided to serve as a current path between a movable conductor in the vacuum casing of the vacuum capacitor and one end side (a metal member) of the vacuum casing and also to divide an inside of the vacuum casing into a vacuum chamber and an atmospheric chamber then to keep the vacuum chamber airtight.
Although also a structure in which a plurality of bellows (e.g. stainless bellows and copper bellows) are used and these bellows are arranged parallel to each other at a certain distance in the vacuum casing has been known (for instance, Patent Document 1), in view of market demand of space-saving, size reduction, etc. nowadays, it is desirable that the bellows having good characteristics (such as mechanical characteristics and electric characteristics) should be used singly.
As a method for improving the characteristics of the bellows, a method for improving the electric characteristics by forming a conductive coating layer on a surface of the bellows-shaped base body (such as stainless steel) by plating (e.g. plating with copper) has been known. In the case where the base body formed from the stainless steel is plated with the copper etc., as a pretreatment process, a surface treatment (a mid-plating process) by nickel plating is performed.
Further, a method, in which bellows-shaped base body formed from beryllium copper is used and a brazing portion of the bellows is plated with copper or plated with nickel, has been also known (for instance, Patent Document 2).
However, the plating process of the bellows-shaped base body requires an expert special process, and consignees also decreases and it is difficult to get the expert special process. In addition, under present circumstances, a manufacturing yield is low, thus the plating process of the bellows-shaped base body is not a high-productivity manner.
Further, since the nickel in itself is magnetic material and the nickel acts as a part of a heat generating source upon power supply in use of the vacuum capacitor or a high frequency power application (a high frequency voltage-application), this could affect characteristics or performance of products. Furthermore, the beryllium copper is inferior in a lifetime characteristic (a metal fatigue rupture characteristic due to continuous expansion and contraction under a heat load) to stainless base material (SUS304L etc.).
In a case where an electroplating process with the copper or silver is performed with the aim of improving a high-frequency characteristic and a high electric conductivity characteristic, its plating thickness (a conductive coating layer thickness) is prone to non-uniform. Further, there is a tendency for the uniformity of the plating thickness to deteriorate with increase of the plating thickness (there is an inverse proportion between the plating thickness and the uniformity of the plating thickness). As a consequence, variations in a total thickness of the whole of the bellows and a mechanical strength distribution occur. For instance, in the case of the use in which the bellows repeatedly expands and contracts like the vacuum capacitor, there is a risk that the metal fatigue rupture will easily occur. Moreover, since a cross-sectional area of the power application (or the voltage application) becomes non-uniform (the cross-sectional area of the power application increases or decreases), there is a possibility that heat value upon the power supply will increase.
For these reasons, studies to readily form the conductive coating layer on the bellows and to make the conductive coating layer thickness as uniform as possible have been done. For example, a cylindrical base body (a base body before being shaped into the bellows) is plated by electroplating, or the conductive coating layer is formed, for instance, by fitting an extremely thin coating tube (a thickness is about 100˜200 μm) onto the base body so as to cover an outer peripheral side of the base body, and after that, by forming the base body into a desired shape, the bellows is obtained. Such method for obtaining the bellows has been studied (for instance, Patent Document 3).
However, even though the base body are molded or shaped after the electroplating in the method, the conductive coating layer thickness is not a little non-uniform (a variation of an average ±10%). Further, in the method in which the coating tube is merely fitted, for instance, a difference in deformation or strain between the base body and the conductive coating layer arises when forming the base body into the desired shape. Thus, in view of the mechanical strength such as the rupture, since this method is an extraordinarily difficult manufacturing process, the realization is low.
Here, although a technical field is different from the bellows of the vacuum capacitor, with regard to a feeding roll, a sheet member is wrapped around a cylindrical base body, and both edges of the sheet member are tacked by welding, then these base body and sheet member are diffusion-bonded. Such technique has been known (for instance, Patent Document 4).
In view of the foregoing, an object of the present invention is to find a different method from the above mentioned method in which after forming the conductive coating layer on the cylindrical base body, the base body is formed into the desired shape, and to obtain the bellows that has the conductive coating layer having a more uniform thickness and has extremely good characteristics (the mechanical characteristics and the electric characteristics).